Two membrane hound enzymes which tautomerize L-dopachrome and are specific for the L-isomer of dopachrome have been defined in melanin forming cells. Another enzyme that tautomerizes D-dopachrome with concomitant decarhoxylation to give 5,6-dihydroxyindole (DHI) was found in the cytoplasm of human melanoma cells, human liver and in all of the organs studied in rat. The decolorization of D-dopachrome with the formation of DHI was used in monitoring the isolation of a tautomerase from liver of male rats and therefore the enzyme is provisionally called D-dopachrome tautomerase. The molecular weight of D-dopachrome tautomerase monomer was approximately 12 kD and its N-terminal amino acid sequence was P-F-V-E-L-E-T-N-L-P-A-. The K_m for D-dopachrome was 1.5 mM and V_max 0.5 mmol per min and mg protein.